E-bay
Outside of that, I can't find much information so you may have to contact Gibson.... from what I've found you may have to find a Licensed Gibson Repairman to get it done .... seen listed roughly $150
I don't know anything about his one.... but THD may make a replacement, maybe worth looking around into and maybe shooting them an email about to know for sure
Link
Sorry if I'm not much help but hopefully a couple more tires for you kick till you find what you need